<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        
        .item {
            display: flex;
            justify-content: center;
            align-items: center;
            width: 100px;
            height: 100px;
            background-color: green;
        }
        
        .item1 {
            margin: 50px auto 0;
        }
        
        .item2 {
            float: left;
            margin-left: 50%;
            position: relative;
            left: -100px;
        }
        
        .item3 {
            float: left;
            position: relative;
            left: -100px;
        }
    </style>
</head>

<body>
    <div class="item item1">
        上
    </div>
    <div class="item item2">
        左
    </div>
    <div class="item item3">
        右
    </div>
</body>

</html>